Precipitation hardening in aluminium alloys provides well-known examples of how identical materials can develop very different microstructures through different processing conditions or the addition of trace alloying elements. Desirable microstructures for useful materials properties such as high mechanical strength often involve the precipitation of metastable phases from a supersaturated solid solution. A textbook case is the precipitation of Guinier-Preston (GP) zones in Al-Cu alloys [1] and the subsequent formation of θ” and θ’ metastable phases [2]. More than 75 years after the original work by Guinier and Preston, the atomic-scale mechanisms behind the solid-solid phase transformations associated with the nucleation and growth of those phases remain unknown. Such an understanding is required if one is to move towards rational design of new high-performance alloys aided by modern computational techniques.

In the last eight years aberration-corrected scanning transmission electron microscopy (STEM) has enabled significant progress in the determination of bulk and interfacial structures of alloy precipitates. Here we present recent work combining STEM observations and computer simulations for the structural and energetics characterisation at the atomic scale of precipitate phases and their interfaces. We will show that even classic binary alloy systems such Al-Cu, Al-Au and Al-Ag can reveal surprising characteristics. For example, the two isostructural phases θ’ (Al2Cu) and η (Al2Au) display vastly different interfacial structures (see Fig. 1) [3-5]. These differences can be explained by the calculated defect energies for Cu and Au solute in aluminium [5]. Another unexpected finding was that of a new intermediate phase, denoted Z, in the Al-Ag system. This phase is coherent with the Al matrix and consists of alternating bilayers of Ag and Al (see Fig. 2) [6]. The structure of the Z phase is analogous to that of Ag segregating on θ’ precipitates (see Fig. 3) [7]. Finally, we will present examples of precipitation pathways being altered through the introduction of certain alloying additions or lattice defects.

The authors acknowledge funding from the Australian Research Council (DP150100558 and LE0454166), computational support from the Monash Sun Grid cluster, the National Computing Infrastructure and Pawsey Supercomputing Centre funded by the Australian Government, and the use of facilities within the Monash Centre for Electron Microscopy.